What's normal ? Have a nose at the sites to see what's on offer. There are various sizes and various powers. The one I have I tend to use on medium power as high/normal just does it too fast. Times that I'm used to, or are printed on the box the food came in, result in my having to mop up the mess spilled over onto the turntable.
It costs much less to cook something in a Microwave than by a regular cooker.
I find it ideal for things that need a good long simmer. I use mine for curries, pasta sauces, etc. You don't have to stand there and stir the pot to stop it from sticking. Marvellous machines !
After microwave shopping myself a little while back I found there is no 'normal' size. Even I they look the same on the outside they were'nt necessarily the same on the inside. Measuring in litres is all very well but 2 models with the same volume may not be the same depth/widthe/height. i ended up going to Currys so I could check them over with a tape measure myself. Having a grill in a microwave can be useful if you don't have a separate grill in your main oven.
Litres is simply the standard measurement for cubic capacity. Years ago the term litres was used alongside cubic centimetres as they effectively mean the same, but the latter terminology fell out of usage, a bit like centigrade did for temperature.
Microwaves aren't measured in inches because they are three dimensional as opposed to TVs where it is the two dimensional flat screen that is defined.